As shown in FIG. 2, a conventional combustion apparatus applied generally to the gas turbine has usually been equipped with a diffusion burner C.sub.1 at the head thereof and with a premix burner C.sub.2 downstream of this diffusion burner as shown in FIG. 2.
Thus, the burners have been combined in two stages for the purpose of obtaining a combustion apparatus capable of effecting a low NOx and stable combustion due to the diffusion combustion which achieves a high combustion stability and the premix combustion which achieves a high reduction of NOx concentration although the stable combustion range therein is narrow.
This type of two-stage combustion apparatus comprises an antechamber or diffusion combustion chamber 1, F.sub.1 nozzles 2 for diffusion combustion, a main chamber or premix combustion chamber 3, F.sub.2 nozzles 4 for premix combustion, and a combustion air supply section. Particularly, in order to control the flow rate of premixed air A.sub.2 through the F.sub.2 nozzles 4 the stable combustion range of which is narrow, an internal flow rate control (IFC), 5 is provided in this combustion air supply section.
In the combustion apparatus having such construction, fuel-air ratio (the ratio of fuel amount to air amount) is controlled so as to be maintained at a constant value by changing the air flow rate in response to the fuel flow rate determined beforehand for the operation of the gas turbine as disclosed in Japanese Patent unexamined Publication No. 60-66020 as well. Namely, the fuel flow rate is changed in accordance with the change of load and the distribution of air in the burners is also changed in accordance with the change of this fuel flow rate so as to set the fuel-to-air ratio at the predetermined value, thereby realizing the stabilization of the combustion and the reduction of the NOx.
FIG. 3 shows the relationship between the fuel-air ratio and the NOx, in which the abscissa represents the fuel-air ratio and the ordinate represents the NOx relative value and which illustrates the difference between the diffusion combustion and the premix combustion. The theoretical fuel-air ratio of methane is 0.058, and gas turbine burners are usually used at fuel-air ratios less than 0.058. In operation, the diffusion combustion which is stable over the wide operation range is utilized from the start-up to the low-load operation of the gas turbine, and the premix combustion and the diffusion combustion are utilized simultaneously from the low-load operation to the rated-load operation, so as to reduce the NOx. On that occasion, since the range of the fuel-air ratio in which the premix combustion is effected completely (operation range of F.sub.2) is narrow, the fuel-air ratio is set to be in the operation range by controlling the flow rate of air A.sub.2 by the IFC 5 of FIG. 2. Total combustion air A of FIG. 2 is divided into a flow rate of air A.sub.1 flowing through the antechamber 1, a flow rate of air A.sub.2 flowing through the main chamber 3 and a flow rate of air A.sub.3 flowing into through dilution holes 6. When the IFC 5 is opened, the amount of air A.sub.2 is increased and the amounts of airs A.sub.1 and A.sub.3 are decreased, while when it is closed, the amount of air A.sub.2 is decreased and the amounts of airs A.sub.1 and A.sub. 3 are increased.
From the viewpoint of efficient use of energy, there arises a problem of the treatment of vaporized gas called BOG gas (boil-off gas) generated in power generation using LNG. The calorific content of the BOG gas is lower than that of the LNG due to the difference in boiling points of the fuel components. If the BOG gas is not discharged to the outside periodically, the internal pressure of an LNG tank is increased to bring about damage. To cope with this, the BOG gas is treated as being mixed with the LNG in the existing circumstances. This results in a sudden change of the calorific value in the LNG power plant. It is therefore difficult to realize the stable combustion and the reduction of the NOx by controlling the fuel flow alone in the conventional manner.
In the above-described prior art, change of the fuel flow rate due to load change has been taken into consideration, and however, change of the fuel characteristics such as the calorific value has not been taken into consideration. Therefore, in case of effecting the lean premix combustion and the two-stage combustion including the diffusion combustion and the premix combustion with the low NOx burner or the like, if the calorific value is changed during the constant fuel flow operation, a combustion flame is made unstable so as to cause a flame-out, or conversely a backfire of the premix flame occurs thereby increasing, increase the production of the NOx and the like. Further, this gives rise to a problem that if the calorific value of the supplied fuel is changed drastically during the constant fuel flow rate operation or the constant load operation, the load is changed greatly. Change of the calorific value of the fuel is the problem caused at the time of treating a low-calorie gas BOG gas, which is vaporized to above the LNG, in the LNG plant.
In order to realize a stable combustion over the wide range from no-load operation to the rated-load operation, such control as shown in FIG. 4 has been performed. In this diagram, the abscissa represents the fuel command signal corresponding to the load ranging from the no load to the rated load and the ordinate represents the flow rate of fuel through the F.sub.1 and F.sub.2 nozzles 2 and 4 and the opening of the IFC 5. On the lower load side, the flow rate of fuel is small and a lean combustion is effected, so that the diffusion combustion is utilized only due to the F.sub.1 nozzles 2. The IFC 5 is fully opened to reduce the air in the ante-chamber 1 so that the fuel-air ratio is increased to realize the stable combustion. It is necessary to reduce the ignition load of the F.sub.2 nozzles 4 for the purpose of expanding the range of the operation load. Therefore, by closing the IFC 5 from the point 1 to the point 2 as shown in FIG. 4, the flow rate of air A.sub.2 through the main chamber 3 is reduced so that the combustion can be effected in a stabilized manner with the fuel supplied from the F.sub.2 nozzles 4 at the time of changing over the fuel. At that time, the flow rate of air A.sub.1 through the antechamber 1 is increased so far as the combustion can be effected in a stabilized manner with the flow rate of the fuel flowing through the F.sub.1 nozzles 2 at the time of the changing-over, and the load at that time is regarded as the minimum change-over load.
After the ignition at the F.sub.2 nozzles 4, the opening of the IFC 5 is increased, as the load is increased to the rated load in due order through A.fwdarw.B.fwdarw.C, so that the flow rate of air A.sub.2 through the main chamber 3 is increased and, at the same time, the fuel supplied from the F.sub.2 nozzles 4 is increased, thereby setting the fuel-air ratio to be in the operation range shown in FIG. 3 so as to reduce the NOx.
In controlling the fuel, the fuel command signal is sent to an IFC opening setting device 7 so as to operate an IFC driving device 8 as shown in FIG. 2.